1. Field of the Invention
This invention relates to transparent coating compositions containing nanoparticles, to a process for their production and to their use for preparing coatings having improved scratch resistance.
2. Description of the Prior Art
The preparation of substantially nanoscale particles in an organic, inorganic or organic/inorganic matrix is of interest in many applications. Specific combinations of properties in coatings, such as transparency and wear resistance, may be obtained by using nanoparticles. It would be desirable, especially for high-grade transparent lacquer applications, to provide lacquer binders with nanoparticles such that, at constant transparency and gloss, an improvement in scratch resistance is obtained.
Various different processes may be considered for the production of preparations containing nanoscale particles.
Several processes are known for the production of dry nanoscale materials, which are used on a large industrial scale primarily in the production of pyrogenic silica (for example, the Aerosil process; the arc process, DE-A 1,180,723; and the plasma process, Powder Technol. 1978, 20, 159). These processes are described, inter alia, in Chemische Technologie, volume 2, Anorganische TechnologieII, 4.sup.th edition, 1983, pg 77.
Further examples of the production of dry materials having primary particles which are nanoscale relate to the production of ceramic particles by matrix pyrolysis processes in a combustible support material as described, e.g., in EP-A 680,454 and EP-A 697,995.
With appropriate process control, the stated processes for the production of nanoscale powders do indeed yield primary particles which are nanoscale (approx. 5 to 50 nm diameter). However, the particles are not in the form of discrete particles, but instead predominantly assume the form of agglomerates due to consolidation of the primary particles. Such agglomerates may reach diameters of several thousand nanometers, such that the desired characteristics associated with the nanoscale nature of the particles cannot be achieved.
The particles may be deagglomerated, for example, by grinding as described in EP-A 637,616. Agglomerates may be reduced to one sixth of their size in this manner. However, the low space/time yield and the unavoidable contamination due to abrasion from the grinding additives are disadvantageous.
Previously, it has not been known how to produce agglomerate-free powders or powder preparations at reasonable cost from the available nanoscale powders containing agglomerates. Various alternative production processes have been developed in which products predominantly containing agglomerate-free nanoscale particles or composites may be produced by means of a controlled growth process starting from discrete low molecular weight starting materials or such materials in, for example, sol form.
It is thus possible using the sol/gel process, starting from metal alkoxides, to produce particles having an average diameter of below 50 nm by a controlled increase in molecular weight. Such systems are used, for example, as coating compositions or active substance precursors as described, e.g., in The Polymeric Materials Encyclopedia1996, volume 6, 4782-4792 et seq.).
Due to the great technical complexity that is generally associated with such production processes, the resultant products may be used in only limited applications. Such processes are also applicable to only a limited selection of different classes of chemicals.
Nanoscale metal oxide sols are also known. These are usually 30 to 50% colloidal solutions of metal oxides (Si, Al, Ti, Zr, Ta, Sn, Zn) having average particle sizes of 4 to around 60 nm in aqueous or organic media. It is possible to prevent such metal oxide sols from agglomerating by electric and/or steric stabilization of the particle surfaces. Aqueous silica sols may in particular be mentioned, which may be produced, for example, from alkaline solutions by ion exchange processes (for example Ullmann's Encyclopedia of Industrial Chemistry, 5.sup.th edition, volume A23, VCH-Verlag, Weinheim, 1993, pp. 614-629). Such products are commercially available, for example under trade names such as Levasil (Bayer AG).
The disadvantage of nanoparticle dispersions, such as silica sols or other metal oxide sols, resides in their strong tendency to agglomerate if the dissolving medium is removed or altered, such that it is not straightforwardly possible to incorporate them homogeneously into a foreign matrix, such as a lacquer (binder) formulation. Homogeneous incorporation into a lacquer (binder) formulation is possible by modifying the surface of the particles and adapting the solvent. Such a process (for example EP-A 768,351) is, however, highly technically complex and applicable only under certain circumstances.
EP-A 766,997 describes a process for the production of finely divided dispersions of solids. Using this process, it is possible to comminute suspended solid particles. The deagglomeration of materials consisting of nanoscale primary particles and their use in lacquer binders was not described in this document. The process is known as a jet dispersion process and is already used industrially for other purposes, such as finely dispersing immiscible liquid phases, such as water in oil. Production of improved two-component aqueous lacquer emulsions by means of finer emulsification may proceed, for example, in accordance with the process of EP-A 685,544.
It is an object of the present invention to provide a simple process for the production of transparent coating compositions, which contain nanoscale particles in order to improve properties, such as scratch resistance.
It has now surprisingly been found that by using the jet dispersion process described in EP-A 766,997 it is possible to bring about a distinct reduction in the agglomerate content of a predispersion of nanoparticles containing agglomerates. The dispersions of solids produced using this process may be used as lacquer binders for transparent coatings. The dispersions of nanoparticles have also proved to be particularly resistant to reagglomeration and settling in the presence of lacquer binders.